setup.py: add Babel to dependencies

Status Job ID Name Coverage
  Build
created #18913
build:2.7
created #18914
build:3
 
  Test
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allowed to fail
lint:2.7
created #18920
allowed to fail
lint:3
created #18915
pyflakes:2.7
created #18919
pyflakes:3
created #18917
unittests:2.7
created #18921
unittests:3
created #18918
allowed to fail
unittests-modules:2.7
created #18922
allowed to fail
unittests-modules:3
 
  Deploy
created #18923
doc:2.7
created #18924
doc:3